Synchronizing signal generator



C. E. HALLMARK SYNCHRONIZING SIGNAL GENERATOR Sept. 17, 1946.

Filed NOV. 18, 1943 utili. 111-41 ww mw INVENTOR t0 mln mi .LA

wvvll' Il l ATTORNEY Patented Sept. 17, 1946 UNITED STATES PATENT OFFICE SYNCHRONIZING SIGNAL GENERATOR Application November 18, 1943, Serial No. 510,759

4 Claims.

This invention relates :to frequency generators and particularly to scanning frequency generators for television systems.

In television systems it is common practice to generate the scanning frequency voltages in such a manner that the frequencies of these voltages are maintained in a predetermined relationship with a standardizing frequency. It generally is convenient to utilize as the standardizing frequency a 60 cycle alternating current derived from a commercial power source. According to the present RMA standards video signals are transmitted at the rate of 30 frames per second. Each frame comprises an odd number of scanning lines and, where a 2 to 1 interlaced Scanning pattern is used, th'e field scanning frequency is 60 cycles per second.

The scanning frequencies usually are derived from an oscillator which is operated either at a high or a low frequency. In the case of a low frequency oscillator its frequency ordinarily is 60 cycles per second and its output voltage is multiplied in frequency rto twice the line scanning frequency. Such an expedient is necessary for the reason that the line scanning frequency for an odd number of scanning lines is not an even harmonic of the 60 cycle oscillator frequency When a 2 to 1 interlaced scanning pattern is used. 'I'he line scanning frequency is derived from the voltage having twice the line scanning frequency by th'e use of a frequency divider. The 60 cycle frequency for the oscillator generally is chosen for the reason that it conforms to the eld scanning frequency and also to the frequency of the standardizing source. Numerous arrangements have been utilized for comparing the 60 cycle oscillator frequency with the 60 cycle alternating current derived from a commercial power source and for utilizing the output of the frequency comparator to control the oscillator frequency so that it will be in agreement at all times with the source of a standardizing frequency.

In an arrangement Wh'ere the oscillator is operated at a high frequency it also is necessary, in order to derive the low frequency scanning voltages and a voltage suitable for Vcomparison withthe standardizing source, to employ a number of frequency dividers.

It is an object of the instant invention, therefore, to provide a novel means `for generatingtelevision scanning voltages definitely` related `in frequency to a standardizing frequency and in which theuse Qf'frequency dividing apparatus is obviated.

Where it is desired `to control the frequency of an oscillator automatically, it is customary to use a so-called LC type of oscillator for 4the reason that a reactance tube may be employed as the frequency varying means. Howeverll it frequently is desirable to use a Slo-called RC type of oscillator but, a device of this character is not susceptible of automatic frequency control by means of a conventional reactance tube.

Another object of th'e invention is to provide a novelautomatic frequency control for an RC type of oscillation generator.

In accordance with the `present invention there is provideda means for producing oscillations of adistortedwave form at a submultiple frequency of the standardizing source. The distorted wave form contains a number of harmonics of the submultiple frequency, one of which is nominally equal to Vthe frequency of the standardizing source. This harmonic and the standardizing frequency are compared'to develop an automatic frequency control `voltage for employment in connection with the oscillator.

The form :of oscillation generator utilized comprises a vacuum tube having the output and input circuits thereof coupled by a phase shifting network for the feedback of output circuit energy to the input circuit. The network comprises a plurality of meshes, each including resistance and reactance of one type only. The resistance element of one of the phase shifting meshes consists of the space discharge path of a vacuum tube. The `magnitude of this resistance element is varied by the automatic frequency control Voltage to vary the total phase shift of the feedback energy and, thus, vary the frequency of oscillation.

For a better understanding of the invention, together with other and Yfurther objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed `out in the appended claims.

In th'e drawing, the single figure isa schematic circuit diagram of a scanning frequency generator and automatic frequency control apparatus embodying the invention.

Having reference `now to the drawing, it is assumed that `it is desired to generate scanning voltages for a -televisionsystem employing an odd number `of scanning lines per frame and 30 framespersecond with a 2 to 1 interlaced scanning pattern. In such a case the frequency of the field scanning 4voltage will be 60 cycles `per second. Also, in this case thesource of standtrated as a pentode. However, it is contemplated Y that other types of tubes may be employed with 'Y equal facility without departing from the spirit or scope of the invention. The cathode ofthe tube I is connected to ground and-the anode is connected through a load inductor 2 to the positive terminal of a source of direct current such as a battery 3, of which thelnegativeterminal ,is-

connected to ground. The suppressor grid 4 of the tube I is connected directly to ground and the screen grid 5 is connected through a resistor 6 to the positive terminal-of the battery 3 and also through a by-pass condenser 'I to ground.

The anode-cf the tube Iis connected to a triple mesh phase shifting network of capacitance and resistance elements.` As illustrated, the network is formed of series condensers and shunt resistors. It is contemplated that a network of series resistors and shunt condensers are within the scope of this invention. Condensers B, 9 and II comprise, respectively, the capacitance elements of the first, secondand third network meshes. The resistance elements of the successive network meshes comprise, respectively, the space discharge path of a triode Vacuum tube I2, andresistors I3 and i4. The network is terminated in a resistor I5, one terminal of which is grounded and the other terminal is connected throughV a resistor I6 to the control grid Il of the oscillator tube I. The control grid also is by-passed to ground by a condenser I8. 4The anodel of the oscillator tube I is coupled by a condenser I8 to the control `grid 2| of a pentode vacuum tube 22 serving as the wave form distorter, A resistor 23, connected between the control grid of the tube 22 and ground,serves as the grid leak resistance for this coupling circuit. The screen grid 24 of the tube 22 is connected through a resistor 25 to the positive terminal of the battery 3 and the suppressor grid 2B is connected directly to ground. The anode of the tube 22 is connected through an inductor 2l, which is by-p-assed by a condenser 28, and through the resistor 25 to the positive terminal of the battery 3. The cathode of the tube is connected through a self-biasing resistor 29 to ground and a condenser 3l is connected to bypass the resistor 23. The self-bias voltage developed by this arrangement is to cause the tube to` operate in the anode current saturation'region.

The apparatus for selecting the low frequency scanning Voltage of 60 cycles per second and the voltage for comparison with the source of standardizing frequency includes a pentode vacuum tube 32. The cathode and the suppressor grid lector tube network includes series condensers 38, 39 and 4I, the latter of which preferably isV adjustable so as to provide for an adjustment in the natural oscillation period of the selector circuit. The shunt elements of the phase shifting network are resistors 42, 43 and 44, the latter of which preferably is in the form of a potentiometerhaving an adjustable contact 45. The contact` 45 is connected through a resistor 46 to the control grid 47! of the selector tube 32. The con.

trol grid of this tube also is by-passed to ground by a condenser 43. Although the phase shifting networks associated with tubes I and 32 are similar, the output energy feedback to the input circuit, in the case of tube I, is suflicient to sustain oscillation at a frequency of Y3!! cycles per second, whereas, in the case of tube 32, the feedback energy is insufcient to sustain oscillation.

33 of this tube are connected directly to ground.

The anode of the tube 32 is connected through a load resistor 34 to the positive terminal of the battery 3. The screen grid 35 is connected through a resistor 36 to the positive terminal of the battery 3 and also is by-passed to ground by It is merely sufficient to enable the tube 32 to act as a lfrequency selective amplifier responsive only to a frequency of cycles per second.

In order to furnish a control voltage for driving the selector tube 32 at the desired frequency of 60 cycles per second, the anode of theY distortion tube 22 is coupled by an arrangement including a condenser 43 and va resistor 5l to the control grid 4l of the selector tube. Preferably, the resistor 5I is in the form of a potentiometer having'an adjustable contact 52 which is connected bya resistor 53 to the potentiometer` contact 45 in the feedback circuit of the selector-tube 32 and also through a resistor 46 to the grid 4l of the tube.

The anode Vof the selector tube 32 is coupled by means of a condenser 54 and a potentiometer 55, having an adjustable contact 56, to the phase comparator. This apparatus includes a diode 51 having an anode which is connected lto ground through a resistor 58 and also to the potentiometer contact 55 through a condenser 59. YThe cathode of the diode 5l is connected to an adjustable contact 6I of a potentiometer 62, of which the terminals are connected to a grounded source of a standard 60 cycle alternating current sourc connected to the terminals 63.

The automatic frequency control Voltage developed by the phase comparator is impressed upon the frequency determining circuit of the oscillator tube I by means of a connection from the anode of the diode 5l through a resistor 64 to the control grid of the triode I2. The control grid of this tube is connected to an adjustable contact 65 of a potentiometer 66 and also through a filter condenser 6l to ground. The terminals of the potentiometer are connectedY to the terminals of a source of biasing voltagesuch as a battery 68.

The anode of the. distortion tube 22 also is coupled to a high frequency selecting cir-cuit 69 by means of a condenser 1B. The selector 69 may be substantially similar to the selector including the pentode tube 32, except that the phase shifting network therefor is designed to operate the high frequency selector at the line scanning frequency.- L

'I'he output circuit of the field scanning frequency selector including .the tube 32, is connected toa Vertical scanning generator 1l. Similarly, the output circuit of the line scanning frequency selector 69 is connected to a horizontal scanning generator 12. The scanning generators `II and 12 lmay be conventional and are not shown in detail herein for the reason that they form no part of the present invention. y

Referring now to the operation of the illustraphase.

atomes tive embodiment of the invention, .assume that the oscillator tube l, together with Vits frequency determining circuit including the series convdensers B, 9 and Il and the shunt triode I2 and resistors I3, ld and l5, effects the generation of a voltage of sinusoidal wave form having a frequency cf 30 cycles per second. This frequency is determined by the parameters of .the vphase shifting meshes connected between the output and input circuits of the tube. Each mesh, including a series condenser, such as 9, and a shunt resistor, such vas I3, effects a Vphase shift, in `the instant embodiment of the invention, of approximately 60 in the voltage of that portion of vthe energy delivered by the `output circuit of the tube to the `frequency Vdetermining circuit. Thus, with the .three disclosed meshes of this character, the voltage of Athis energy is shifted 180 in This phase-shifted energy is impressed upon the input circuit of the tube I in suiicien-t magnitude .to produce `sustained oscillations in the output circuit. The oscillator frequency may be varied by adjusting one or more of the freqency determining circuit parameters, for example, the value of one of the shunt resistors. A modification of the shunt resistance consisting of the triode l2 will effect a change in the amount of phase shift produced by the first mesh. Consequently, a similar change will be effected in the total phase shift, thereby .to change the oscillation frequency.

The `.sine Wave voltage derived from the oscillator tube l is converted by the distortion tube 22 into a substantially rectangular wave form voltage having a frequency of 30 cycles per second. The rectangular Wave form voltage is produced by driving the tube to saturation through the use of the self-biasing resistor 29. A rectangular Waveform `is referred .to merely for illustrative purposes, Substantially any distorted wave form containing the desired harmonics will serve with equal facility.

.A portion of the energy of the rectangular wave form voltage is employed to drive the low frequency selector, including .the tube 32, in e, manner to develop in the output circuit of this tube a substantially sinusoidal voltage having a frequency of 60 cycles per second which is the second harmonic of the generated frequency. The natural period of the network including the series condensers 38, 39 and 4l and ythe shun-t resistors 42, 43 and 44 is substantially 60 cycles per second. By means of the adjustment of the potentiometer contact 45 the magnitude ofthe phase-shifted energy developed in the network is maintained below the value necessary to sustain oscillations in the output circuit of the tube 32. This energy is combined with enough 60 cycle energy derived from the rectangular wave form voltage, as determined by the adjustment of .the potentiometer contact 52, to produce feedback energy at 60 cycles per second of suiicient magnitude to drive the tube 32 at this frequency.

The phase comparator, including the diode 51, functions to detect deviations of the 60 cycle derived frequency from .the standard 60 cycle frequency. These frequency deviations are manifested as phase displacements of .the derived voltage wave with respect to the standardizing voltage wave. iThe voltage of the diode cathode varies sinusoidally under the control of the standardizing frequency voltage derived from the potentiometer 62. The voltage of the diode anode also varies sinusoidally under the control of the oscillator derived voltage impressed upon it by the condenser 50. .So long as the cathode and anode voltages arein phase the anode-to-cathotde potential of the diode remains constantxand vthe current conduction .through .the .diodeandth'e 1resistor 58 is'maintainedata .constant valuew'hich is dependent upon the relative magnitudes :of the anode and cathode voltages. `A phase deviation of .these `voltages effects a corresponding change in the diode anode-.to-cathode potential and, consequentlyin .the value of .the current conduction through `the diode and the resistor 58. Thus, a unidirectional Voltageis developed in the resistor of a magnitude Vcorresponding `to the'rdegree of the phase deviation.

This unidirectional voltage is combined with the biasing voltage derived from the potentiometer 66 to form an `automatic frequency `control voltage. The magnitude and polarity of this control voltage represents the instantaneousphase deviations of the oscillator derived 60 cycle volt-V age from the voltage derived from the standardizing source. The control voltage is` impressed upon the grid of the triode l2 in the manner described. Any alternating current component which may be present in the voltage derived from the output circuit of the diode 5l is substantially eliminated from the frequency control voltage `by the lter condenser 6l. The control voltage impressed upon the grid of the triode l2 effectively varies the impedance of the space discharge `path of this tube. A variation in this impedance effects a corresponding variation in the degree of phase shift produced in the first mesh' of the phase shifting network connected between the output and input circuits of the oscillator tube i. Thus, the phase of the feedback energy impressed upon the input circuit of the oscillator tube is altered in one sense or the other, thereby to effect a corresponding change in the frequency of the generated voltage.

The frequency of the voltage developed in `the output circuit of the low frequency selector tube 32, being 60 cycles per second, is suitable :for controlling the generation of the field scanning voltage since it is desired that this voltage @be `generated at this frequency. The `harmonic of the generated 30 cycle voltage, which the highfrequency selector G9 derives from the rectangular 30 cycle wave developed in the output circuit of the distortion tube 22, produces a Voltage in the output circuit of the high frequency selector having the desired line scanning frequency.

While it will be understood that the circuit specifications of a scanning frequency generator embodying this invention may vary according to the frequency requirements of any particular television system, the following circuit specifications for a generator employed to develop voltages at 60 cycles per second and 15,750 cycles per second are included, by Way of example only:

Vacuum tubes 1 and 32 GAC?? Inductor 2 henries" `100 Battery 3 vo1ts 250 Resistors 6 and 36 ohms 82,000 Condenser 7 microfarads 35 Condensers 8, 9 and 28 do 0.17 Condenser 11 do 0.20 Vacuum tube 12 l 6C8G Resistors 13 ,14, 15, 34, 42, 43, 44 and 53 ohnis 10,000 Resistors 16 and 46 do 330 Condensers 18 and 48 -micro-n1icrofarads 200 Condenser 19 microfarads-- 4 Vacuum tube 22 6SJ7 7 Resistors 23, 58 and 66.A megohm-- 1 Resistor 25 ohms 50,000` Inductor 27 henries-- 17 Resistor 29 ohms 25,000 Condenser 31 microfarads, 0.5 Condenser 37 do 40 Condensers 38 and 39 do 0.05 Condenser 41 do 0.132 Condensersl49- and 70 do 0.5 Resistors 51 and 55 ol1ms 100,000 Condenser 54 microfarads-- 1 Vacuum tube 57 6H6 Condenser 59 microfarads-- 0.1 Resistor 62 ohms 1,000 Resistor 64 do 230,000 Condenser 67 microfarads 0.1 Battery 68 volts 12 It is evident that a scanning frequency generator in accordance With the instant invention possesses a number of advantageous features not previously realizable in prior art systems. Since the line scanning frequency and the field scanning frequency are not related harmonically in a system, where an odd number of scanning lines per frame and an even number of interlaced fields per frame are necessary, the generation of a sub-harmonic of the field scanning frequency makes it possible to dispense With the multiplicity of frequency dividers and/or frequency multipliers required in prior art systems. Also, when using an oscillator of the type described, the application of an automatic frequency control voltage thereto in the novel manner disclosed makes it possible to positively lock the frequency of the oscillator in a predetermined relationship to a source of standardizing frequency.

K While there has been described what, at present, is considered the preferred embodiment of the invention, itvwill be obvious to those skilled in the art that various changes and modifications may be made therein Without departing from the invention, and therefore, it is aimed in the appended claims to cover all such changes and modications as'fall within the true spirit and scope of the invention.

What is claimed is:

1. In a scanning lfrequency generator for a tele- Vision system, means for producing oscillations of a distorted Wave for-m at a submultiple frequency of a low frequency eld scanning voltage and containing a plurality of harmonics of said submultiple frequency, means coupled to said oscillation producing means and controlled by a har monicof said distorted Wave form oscillations to produce a deecting voltage for said Vtelevision system at field scanning frequency, and means coupled to said oscillation producing means and controlled by another harmonic of said distorted Wave form oscillations to produce a deflecting voltage for said television system at line scanning frequency. A

2. In a scanning frequency generator for a television system, Ymeans for `producing oscillations of a distorted wave for-m at a submultiple frequency of a low frequency eld scanning voltage and containing a plurality of harmonics of said submultiple frequency, means including a low frequency selector coupled to said oscillation producing means to produce a deilecting voltage for said television system at field scanning frequency, and means including a high frequency selector coupled to said oscillation producing means to produce a deflecting voltage for said television systemat line scanning frequency.

3. In a scanning frequency generator for a television system, an electronic oscillation generator for producingsinusoidal oscillations at a submultiple frequency of a low frequency scanning voltage, means for converting said sinusoidal oscillations into oscillations of a substantially rectangular Wave form, said rectangular oscillations being produced at said submultiple frequency and containing substantially all harmonics of said submultiple frequency, a low frequency selector coupled to said Wave form converting. means to produce a control Voltage at the low scanning frequency, a generator coupled to said lowfrequency selector for control thereby to produce a deflecting Voltage for said television system at eld scanning frequency, a high frequency selector coupled to said wave form Vconverting means to producea control voltage at the high scanning frequency, and another voltage generator coupled to said high frequency selector Vfor control thereby to produce a deflecting volta-ge for said television system at line scanning frequency.

4. In a scanning frequency generator for a television system, an electronic oscillation generator for producing sinusoidal oscillations at a submultiple frequency of a low frequency scanning voltage, a distorting vacuum tube for converting said sinusoidal oscillations into oscillationsof a substantially rectangular wave form, said rectangular oscillations beingproduced at said submulj tiple frequency and containing .substantially all harmonicsof said submultiple frequency, a loW frequency selector including a driven oscillator coupled tothe output circuit of said Ydistorting tube to produce a control voltage at the low scanning frequency, the natural period of oscillation of said low frequency selector oscillatorbeing equal to the low frequency scanning voltage, a vertical scanning voltage generator coupled to said 10W frequency selector for control by said low frequency control Voltage to produce a deecting voltage for said television system at field scanning frequency, a high frequency selector coupled to the output circuit of said distorting tube to produce a control voltage at the high scanning frequency, and a horizontal scanning voltage generator coupled to said high frequency selector for control Iby said high frequency control voltage to produce a deflecting voltage for said television system at line scanning frequency.

CLYDE E. HALLMARK. 

